The use of titanium alloys in aerospace construction is of significant interest. Titanium alloys have superior strength-to-weight ratios. Densities range between 4.4 and 4.85 gm/cm.sup.3 and yield strengths from 172 MPa to 1,880 MPa for different titanium alloys. This combination of high strength and low density results in exceptionally favorable strength-to-weight ratios. These ratios are superior to almost all other metals in the range of temperatures reached in the compressors of aircraft gas turbines making the use of titanium very desirable for weight saving.
In addition to high strength-to-weight ratios, titanium alloys possess excellent erosion resistance, high heat transfer efficiency and good corrosion resistance in most environments. Like stainless steel, in the presence of air titanium alloys form a tightly adherent oxide scale that is self healing and severely reduces material loss due to erosion/corrosion.
In airframe/turbine engine applications, it is desirable to replace ferrous alloys including stainless steels, and some nickel based alloys with titanium alloys to save weight. Presently, titanium alloys account for 7% of the weight of airframe structures for commercial aircraft and 20-25% of weight in such structures used for military applications. Uses include bulkheads, air ducting, fairings, keels, and fuselage panels. In addition, casings and larger structures made by electron beam welding plates of various titanium alloys can be substituted for ferrous materials.
One obstacle to greater utilization of titanium within airframes is its reactivity with hot oils and hydraulic fluids. In aircraft, excess heat, age or catalytic reaction can cause lubricants and hydraulic fluids to decompose to acidic materials which quickly attack titanium structures. One major manufacturer of commercial aircraft has noted that phosphate ester based hydraulic fluid has little effect on most metals up to about 115.degree. C. (240.degree. F.), but that titanium alloys can be severely etched, pitted, and embrittled when exposed to such a fluid at temperatures above 132.degree. C. (270.degree. F.). This corrosion occurs on all of the titanium alloys that could be used in airframe construction. Similar effects are seen on corrosion-resistant steel, often known as CRES, at temperatures above 204.degree. C. (400.degree. F.). Temperatures greater than 132.degree. C. can be generated in a variety of airframe locations--for example, in braking systems and in/near engines and support pylons.
Of the hydraulic fluids and oils most commonly used, phosphate ester type synthetic hydraulic fluid exhibits the most rapid attack on airframe materials. This fluid is used in aircraft because it is fire resistant, e.g., it has a high autoignition temperature and shows little tendency to propagate a flame. In addition to attacking titanium (and steels), it strips most finishes from metals and attacks other organic polymer structures. This phosphate ester hydraulic fluid typically contains dibutyl phenyl phosphate and tributyl phosphate. Typical hydraulic fluids of this type are Skydrol 500B and Skydrol LD manufactured by Monsanto Chemical Company.
When this fluid drips on titanium alloys that are operating at a temperature greater than 132.degree. C. (270.degree. F.), black decomposition products accumulate which contain acidic products such as acid phosphates which rapidly attack the titanium structure.
In the past, use of titanium structures in any area exposed to such hydraulic fluid was forbidden since there was no known way of controlling the attack of the hydraulic fluid on the hot component. The temperature of exposure can be as high as 260.degree. C. (500.degree. F.) and can be produced by cool hydraulic fluid dripping on a hot component or hot hydraulic fluid dripping on a cool component. Because the attack produces embrittlement and pitting, protection must be total since embrittlement and pitting both lead to cracks and catastrophic failure.
Applicant has been quite active in developing and patenting various coating and bonding compositions which are highly suitable for coating various surfaces, particularly metal, to impart protective or other characteristics thereto or which may be used as compositions for bonding two surfaces together. Examples of these compositions are disclosed in the U.S. Pat. Nos. 3,248,251 to Allen, issued Apr. 26, 1966, 4,537,632 issued Aug. 27, 1985, and 4,606,967 issued Aug. 19, 1986, both to Mosser, and U.S. Pat. Nos. 4,617,056, issued Oct. 15, 1986, 4,659,613, issued Apr. 21, 1987, and 4,724,172, issued Feb. 9, 1988, all to Mosser and McMordie. All of the aforementioned patents are assigned to the assignee of the present invention. However, none of the aforementioned patents or other prior art known to applicant addresses the problem inherent in coatings of titanium structures which are exposed to hydraulic fluids at extremely high temperatures. Further, neither applicant nor others skilled in the art have previously been able to provide coatings for controlling the attack of the hydraulic fluid on hot components made from titanium or titanium alloys.
Applicant herein provides a coating, a method of coating an article, and a coated article, all related to titanium and titanium alloys as substrates, which now allows such coated substrates to be used in what was previously forbidden in environments.